Hi & welcome to TSF
From what you have had done, it sounds as if you are experiencing interference from the fan controller unit. I believe that it is, in all probability, an electronic controller. Is is very old? Can you get a replacement at a reasonable cost?
I cannot be absolutely sure that this is the cause but it is most likely to be.
Do you know a friendly radio ham? If so, give him a call and explain that you may be having "RF interference" that may be emanating from the controller and can he help in confirming it.
A radio ham will have the knowledge to 'sense/sniff' out the source.